C#: Try fallback dotnet restore without nuget.config

[tamasvajk]

This PR slightly changes the package restore logic in the dependency manager. When a package couldn't be restored through sln or csproj files, we fall back to a dedicated dotnet restore on the package name. This fallback restore uses the nuget.config file in the repo. This PR changes the fallback logic to first try restoring with the nuget.config, and if that fails with NU1301 error, then the restore is tried again without the nuget.config.

[michaelnebel]

This is just a suggestion and nothing that should block merging: Since we are now adding a second out parameter (which contains all output lines) and since we have several ignores of the list of assets files (which is calculated eagerly in the call to dotnet.RestoreProjectToDirectory) it might make sense to introduce a new class to carry the result of a "restore" call.
Maybe something like (RestoreResult should be the return type of RestoreProjectToDictionary and RestoreSolutionToDictionary):

        public class RestoreResult
        {
            private readonly IList<string> output;
            public bool Success { get; }

            public RestoreResult(bool success, IList<string>? output)
            {
                Success = success;
                this.output = output ?? Array.Empty<string>();
            }

            private static IEnumerable<string> GetFirstGroupOnMatch(Regex regex, IEnumerable<string> lines) =>
                lines
                    .Select(line => regex.Match(line))
                    .Where(match => match.Success)
                    .Select(match => match.Groups[1].Value);

            public IEnumerable<string> GetAssetsFilePaths() => GetFirstGroupOnMatch(AssetsFileRegex(), output);

            public IEnumerable<string> GetRestoredProjects() => GetFirstGroupOnMatch(RestoredProjectRegex(), output);

            public bool HasNugetError() => output.Any(s => s.Contains("NU1301"));
        }

[tamasvajk]

Good idea, I'll do it in a followup PR.

[michaelnebel]

Looks good to me!
I have added a suggestion for a possible refactor.